In FIG. 3 of U.S. Pat. No. 4,986,009, a dryer section is disclosed. The purpose of such a dryer section is to dry a fiber web, in particular within a paper-making machine having a very high operating speed. The maximum operating speed may be about 1,500 m/min or even higher.
Critical points of such a dryer section are:
1. The area where the fiber web is transferred from one dryer group to the next dryer group.
2. The so-called departure points where the fiber web and the support belt depart from the drying cylinders.
In the above-mentioned FIG. 3, for transferring the web from a first to a second dryer group, a first suction roll of the second dryer group has the function of a pick-up roll (75). The support belt (70) of the first dryer group travels around a last suction roll (74) and then tangentially to the periphery of the pick-up roll (75) around which the support belt of the second dryer group travels. Upstream of pick-up roll (75), the two support belts (70 and 80) are forming a so-called convergence angle which may be, e.g. between 3.degree. and 30.degree..
This configuration disclosed in FIG. 3 is preferred to that of FIG. 1 of the above-mentioned U.S. patent. In FIG. 1, the pick-up roll is designated (24a) upstream of which the two support belts are traveling parallel (from roll 24 to the roll 24a). In this configuration the fiber web may be subjected to stress, if the two support belts must travel at a certain differential speed.
The high operating speed mentioned above is obtainable, among other reasons, due to the suction rolls since the fiber web is held by suction against the support belt when it travels over the suction rolls, against the centrifugal force exerted on the fiber web. In the area, where the fiber web and the support belt are traveling from the periphery of the so-called delivering drying cylinder onto the periphery of the following suction roll, the fiber web should also be safely held against the support belt. To accomplish this goal, it is known from international publication WO 83/00514, FIG. 2 , to provide a very short distance between the periphery of the suction roll and the peripheries of the adjacent drying cylinders. However, a problem may arise from the fact that the suction roll is positioned symmetrically with respect to the two adjacent drying cylinders: in some cases, an air blow box may be arranged on the periphery of the suction roll, preferably covering only the second half of the zone looped by the support belt (as disclosed in FIG. 3 of the above-mentioned U.S. patent). This may result in an unfavorable small distance between the air blow box and the periphery of the adjacent drying cylinder.
It is a general object of the invention to improve the runability of the dryer section (allowing an extremely high operating speed and avoiding web breaks) while maintaining a high drying efficiency.
It is a further object of the invention to improve the function of the pick-up roll such that the fiber web is safely transferred from one dryer group to the next, permitting a very high operating speed and avoiding any stress subjected to the fiber web. To accomplish this, according to a first aspect of the invention, the second support belt comes into contact with the first support belt only within a small portion of the periphery of the pick-up roll. In other words, a small portion of the periphery of the pick-up roll is wrapped by the support belt of the first dryer group. Preferably, the angle of this periphery portion is selectable during operating of the machine.
It is a further object of the invention to provide an open web draw or open transfer between adjacent dryer groups in order to allow a small web speed differential between the two groups. This results from a further aspect of the invention wherein the support belt is adjustable so that it is out of contact with the periphery of the pickup roll.
It is a further object of the invention to provide a configuration which guarantees holding the fiber web against the support belt when it travels from one of the drying cylinders to the following suction roll while an air blow box may be arranged on the periphery of the suction roll, preferably in the second half of the zone wrapped by the support belt and/or while a certain space should be maintained where vapor escapes from the web before the web comes into contact with the next cylinder.